Automatic focus functions are installed in most of camera modules mounted in general digital cameras, mobile telephones, and smartphones that are the multifunctional mobile telephones, having high compatibility with the Internet and being produced with the basis of the personal-computer functions. The automatic focus functions installed in such compact cameras adopt contrast detection methods, in many cases. The contrast detection method is a method of moving an optical lens actually, detecting the lens position where the contrast of the subject in a captured image is maximized, and moving the lens to the position.
Such a contrast detection method is achievable at lower costs than that of an active method of irradiating infrared rays or ultrasonic waves to the subject and measuring the distance to the subject from the reflected wave. However, there is a problem that it takes time to search for the lens position where the contrast of the subject is maximized. After a user presses the shutter button halfway, it is expected that a process of focusing on the subject is completed for equal to or less than 1 second.
The pixel numbers of the camera modules installed in the general digital cameras or mobile telephones are increasing year by year, and high-definition images can be taken by even these compact cameras. In an extremely precise image, defocusing is easily outstanding, and more highly precise automatic focus control is demanded.
In addition, generally speaking, the devices in which input signals and displacements in accordance with the input signals are represented by a linear function are referred to as linear motion devices. These types of linear motion devices include, for example, automatic focus lenses of cameras.
FIG. 1 is a configuration diagram illustrative of a controller of a linear motion device of one technique. A controller of a linear motion device 12 illustrated in FIG. 1 includes a magnetic field sensor 13, a differential amplifier 14, a non-inverting output buffer 15, an inverting output buffer 16, a first output driver 17, and a second output driver 18. The linear motion device 12 is feedback-controlled by the controller, and includes a lens 9 and a magnet 10.
The magnetic field sensor 13 generates a signal based upon a detected magnetic field to output it as an output signal SA. The output signal SA from the magnetic field sensor 13 and a device position instruction signal SB are input into a positive input terminal and a negative input terminal of the differential amplifier 14, respectively. A manipulated amount signal SC representing manipulated amounts (i.e., the product of deviation and the amplification degree) of output drivers 17 and 18 are output from the differential amplifier 14, to which the output signal SA from the magnetic field sensor 13 are input and the device position instruction signal SB.
The direction and amount of an electric current flowing across the coil 11 of the linear motion device 12 change according to the magnitude of the manipulated amount signal SC. The position of the linear motion device 12 including the magnet 10 changes (i.e., moves) according to the electric current flowing across the coil 11. In this situation, the output signal SA from the magnetic field sensor 13 changes in response to the movement of the magnet 10. The controller detects the position of the linear motion device 12 according to a change in the output signal SA, and performs the feedback control so that the position conforms with the position indicated by the device position instruction signal SB input from the outside.
In the linear motion device 12 illustrated in FIG. 1, here, variations in magnetization of the magnet 10 may occur. Besides, in the controller, variations in the mounting position of the magnetic field sensor 13 may occur. With these variations, the relationship between the position of the linear motion device 12 and the magnetic field detected by the magnetic field sensor 13 differ from the relationship assumed at the time of design.
FIG. 2 is a view illustrative of a relationship between a magnetic field detected by the magnetic field sensor illustrated in FIG. 1 and a position of the linear motion device. In FIG. 2, the vertical axis on the left side represents the magnetic field (hereinafter, referred to as detected magnetic field) detected by magnetic field sensor 13, and vertical axis on the right side represents values of the output signal SA from the magnetic field sensor 13. In addition, the horizontal axis of FIG. 2 represents positions of the linear motion device 12.
The solid line “a” in FIG. 2 indicates a characteristic in a case where there is no misalignment between the detected magnetic field and the position of the linear motion device 12 (just as the design value) for comparison. The dashed line “b” indicates a characteristic in a case where there is a misalignment between the detected magnetic field and the position of the linear motion device 12.
As illustrated in FIG. 2, in the case where there is a variation in magnetization of the magnet 10 or a positional misalignment of the magnetic field sensor 13, the detected magnetic field does not indicate the correct position of the linear motion device 12. For this reason, the controller is not capable of controlling the position of the linear motion device 12 appropriately, in some cases.
In other words, when the linear motion device 12 moves from an end point XBOT to another end point XTOP in the case of being just as the design value as indicated by the solid line “a”, the output signal SA from the magnetic field sensor 13 changes from VMLa to VMHa (in FIG. 2, this range is represented by SA(a)). In this situation, the device position instruction signal SB ranging from VMLa to VMHa, which is a voltage range same as that of the output signal SA from the magnetic field sensor 13, is input to the controller. Then, when the device position instruction signal SB of an intermediate potential VMM (=(VMHa−VMLa)/2+VMLa) is input, the linear motion device 12 obtains an intermediate position XMID.
On the other hand, in the case where there is a variation in magnetization of the magnet 10 or the positional misalignment of the magnetic field sensor 13, the output signal SA from the magnetic field sensor 13 changes from VMLb to VMHb at an inclination different from that of the solid line “a” (in FIG. 2, an alternate long and short dash line “b” having an inclination different from that of the solid line “a” is indicated and a range of this change is represented as SA (b)). Then, when the device position instruction signal SB of the potential VMM (=(VMHa−VMLa)/2+VMLa) is input to controller, the linear motion device 12 is to be located at the position XPOS. There is a problem that the controller is not capable of controlling the position of the linear motion device 12 correctly.
In order to solve the above problem, there is a technique that the output signal SA from the magnetic field sensor 13 and the device position instruction signal SB are synchronized with each other by correcting the output signal SA from the magnetic field sensor 13 or the device position instruction signal SB (for example, see the patent literature 1).
Additionally, the patent literature 2 is directed to a focus control circuit that determines the focal position by moving the lens actually. In the focus control circuit installed in an imaging device including a lens, a driver element for adjusting the position of the lens and a position detecting element for detecting the position of the lens, there are provided with an equalizer for generating a driving signal for aligning the lens position with a target position based upon a difference between the lens position specified by the output signal from the position detecting element and the target position of the lens set from the outside, to output the driving signal to the driver element, and an adjusting circuit for adjusting at least one of a gain or an offset of the position detecting element.
Furthermore, the patent literature 3 discloses a position signal correction circuit of a voice coil motor driving device, including an adder for receiving a position signal indicative of a position detecting sensor according to a sensor signal output from the position detecting sensor provided at a center portion or in the vicinity of the coil of a voice coil motor, and outputting a control signal for controlling the driving of the voice coil motor, and a signal attenuator for attenuating the control signal output from the adder, so that the adder adds the position signal and a reversed phase of the attenuated control signal input from the signal attenuator to output as the control signal.